The invention relates to the field of power systems. More particularly, the present invention relates to maximum power tracking in solar power systems.
Peak power trackers have been used for tracking power delivered to a load from a power source. Peak power trackers are disclosed in U.S. Pat. No. 5,493,204 issued to Caldwell on Feb. 20, 1996, and in U.S. Pat. No. 4,794,272 issued to Bavaro on Dec. 27, 1988. These maximum peak power trackers can be applied to solar arrays providing power to a load where the peak power trackers track the power being delivered from the solar arrays and adjusts operating parameters to maximize the amount of power delivered from the solar array for powering the load. Shared bus current sharing for current mode DC-DC converters is disclosed in U.S. Pat. No. 6,009,000 issued to Siri on Dec. 28, 1999. The shared bus is used for equalizing power delivered through a plurality of converters that convert solar array power into regulated power for powering the load.
The apparatus of Caldwell fails to operate consistently over wide operating ranges depending on the solar array voltage and the current power operating point. The solar array voltage is detected by capacitive differentiation for controlling the duty cycle of a pulse width modulated control signal. Normally, as the solar array voltage increases, the capacitive differentiation voltage increases producing an increase in a pulse width modulation duty cycle increasing the power operation point until passing the peak power point. As the solar array voltage decreases, the capacitive differentiation voltage decreases causing a decrease in the duty cycle providing a decrease in the power operating point. The duty cycle increases and decreases to operate the apparatus dithering about the peak power point. The apparatus fails to function when the solar array voltage has settled at a low voltage when the apparatus operates below the peak power point in a low voltage trapped state. The capacitive differentiation method cannot absolutely detect the slow changes in the solar array voltage after the solar array voltage becomes steady far below the peak power point on the lower voltage side of the peak power point in a power versus array voltage curve. The apparatus may fail to operate at the peak power point when the solar power initially increases from zero after a black out. The solar array voltage initially starts far below the peak power point level on the lower voltage side of the peak power point in the power versus voltage curve profile. In the trapped state, when the solar array voltage changes slowly, the capacitive differentiation method may fail to detect small voltage changes. When the array voltage is low, increasing the duty cycle draws additional current from the solar array tending to further reduce and collapse the array voltage into the low voltage trapped state.
The apparatus of Caldwell also has inherent instability. The apparatus may not function at or near the specific peak power point when the solar array has a high amount of voltage ripple as the apparatus dithers about the peak power point. As the solar array characteristics widely change due to aging and environmental factors. The peak power point and amount of ripple also changes significantly over varying conditions. The Caldwell apparatus does not ensure that the solar array voltage ripple around the peak power point can be controlled to be negligible as compared to the average value of the operating array voltage. Because the array source and load conditions vary, the array voltage ripple around the operating value is unpredictably large. Due to lack of a precise control to limit the array voltage ripple, a large filtering capacitor reduces the ripple, but a large filtering capacitor causes slow changes in the solar array voltage undetectable by capacitive differentiation. A large amount of unpredictable ripple can not be effectively reduced by a fixed value capacitor. Hence, the apparatus cannot sustain the stability of the array voltage within a predetermined ripple amplitude.
Bavaro teaches a peak power tracker that is applied to a stand-alone single DC-DC converter. The use of several power converters could be used to increase the amount of power delivered but would necessarily require additional control circuitry, and, the power delivered to the load may be unequal amongst the converters. Also, the peak power tracker uses a dither signal having a predetermined dither frequency operating in the presence of differing operating conditions. The use of the dither signal avoids initial low voltage trapping. The peak power tracker also uses a second order band-pass filter to detect the converter output current at the dither frequency. The dither signal is compared to the output current signal for controlling the peak power tracker. In practice, a second-order band-pass filter may not be precisely tuned to a center frequency at the dither frequency resulting in operation off the peak power point.
A slow varying control voltage signal modulates a pulse width modulated signal for controlling the DC-DC converter. The dither signal is coupled with the solar array voltage so that very small changes in the solar array voltage can be detected for controlled operation at the peak power point. However, small fluctuations of the pulse width modulation control signal does not guarantee insignificant ripple of the solar array voltage at all conditions under the sun illuminations because the solar array voltage is not effectively regulated over varied amounts of sun illumination and temperature. The peak power tracker cannot regulate the array voltage ripple to a predictable amplitude. The peak power tracker does not regulate the array voltage disadvantageously resulting in unregulated array voltage ripple. The slow pulse width modulation control signal has only two states, increasing and decreasing states and fails to provide regulated solar array voltages during steady state conditions with reduced ripple. The solar array voltage ripple can disadvantageously cause excessive and uncontrolled and imprecise operational dithering about the peak power point. These and other disadvantages are solved or reduced using the invention.
An object of the invention is to provide maximum power from a power source to a load using a DC-DC power converter.
Another object of the invention is to provide maximum power from a power source to a load using a plurality of power converters providing equal contribution in converted power.
Yet another object of the invention is to provide a system for connecting a plurality of converters in parallel for delivering power from a power source for powering a load.
Still another object of the invention is to provide a plurality of DC-DC converters in a power system with maximum peak power operation for converting power from a solar array power source for powering a load.
A further object of the invention is to provide bus stabilization used in a peak power tracking control loop for improved maximum power tracking during over demands by a load and regulated power during under demands by the load.
Yet a further object of the invention is to provide a common dither signal for stabilizing maximum power tracking amongst a plurality of power converters for converting maximum power from a power source for powering a load.
The invention is directed to a maximum power tracker system for providing maximum peak power delivered from a power source to a load. The power source may be as a solar cell array. The maximum power tracker system provides processing management to deliver maximum power to the load depending on available power from the solar array source or sources of similar characteristics. The maximum power tracker system can deliver power to a large class of loads without clamping an output voltage to an optional load battery connected to the load as a backup power source. An output filter capacitor, coupled across the load, is sufficiently large to filter voltage ripples for voltage stabilization around the selected dither frequency. The maximum power tracker system can be applied to a constant power load by using a bus stabilizer coupled across the output voltage. At a frequency above the center frequency of the bus stabilizer, damping effects are created to sufficiently ensure output voltage stability without oscillations due to negative resistance characteristics of the constant power load. The maximum power tracker system can be used to support existing standardized DC-DC converters that use current-mode control in an innermost control loop. For example, parallel-connected current mode DC-DC converters will operate normally in an output voltage regulation mode until the available power from the solar array drops below the load demand when the output voltage loses regulation. When the output voltage is below the regulation level, the maximum power tracking mode of operation is automatically activated and consequently sustains the output voltage below but closest to the regulated level for supplying maximum power to the load.
The maximum power tracker system uses two two-state sample and hold circuits and two lowpass filters to detect the changes in the sampled array input voltages and the load current dithered at a dithered frequency. The rate of change in the load current at low frequency is in the same direction of the change in power drawn from the array so that the load current is an indication of available array power. The array power and voltage signal are used to control the DC-DC converters for maximum power tracking. The lowpass filters in a maximum power tracker have a broad bandwidth suitable for dither frequency detection operation in the presence of loose tolerances of component values selected during manufacturing. The corner frequency of the lowpass filter is above the dither frequency but significantly below one half of the sampling frequency of the two-state sample-and-hold circuits to prevent aliases effect. The first order lowpass filters can be consistently manufactured.
The maximum power tracker uses synchronized maximum power tracking for different solar array sources by using a common dither signal for ease of system control and improved performance. The maximum power tracker utilizes a solar array voltage regulation control mode that regulates the solar array voltage to a predetermined set point that changes very slowly as compared to the closed loop dynamics. The array voltage regulation control ensures that the solar array source observes the DC-DC converter as a resistor load at low frequencies without negative impedance despite a constant power load terminated across the converter output. The maximum power tracker control circuit generates the set point signal to control the array voltage. At any time, the set point signal is in an increasing slowly, decreasing slowly, or constant maintaining state. The set point signal smoothly varies the array voltage using an array regulation control loop without ripple instability. The solar array voltage control loop has a fast response time to ensure the reliable solar array voltage regulation. The fast response of the solar array voltage control loop is achieved using an input bus stabilizer terminated across the input of the current mode DC-DC converter. The input bus stabilizer significantly attenuates resonant peaking introduced by the line filter inductance and net capacitance across the converter input including the solar array capacitance so that the crossover frequency at 0 dB of the array control loop gain extends between 5 kHz to 15 kHZ unity loop gain bandwidth for efficient array voltage stabilization.
The maximum power tracker provides controlled tracking around the peak power using the small dither signal, such as a 0.1 volt. 500 Hz signal, that is superimposed on the set point signal prior to feeding the composite signal to the array voltage control loop as a commanding reference signal. The dither signal has small amplitude at a much lower frequency as compared to the array control loop crossover frequency. The dither signal ensures that the array voltage is regulated to the set point signal so that the solar array voltage ripple is controlled to a predetermined amplitude and frequency in the presence of changes in the peak power point conditions due to the varied sun intensity and temperature imposed on the solar array. The maximum power tracker uses the slow changing set point signal defined by the different operating states including the increasing state, the decreasing state, and the constant maintaining state, the later of which for maintaining maximum power tracking in the presence of varying amounts of available power from the solar arrays and varying amounts of demands from the load.
Near uniform current sharing amongst the plurality of parallel connected DC-DC converters is achieved without conflicts in output voltage regulation using a shared bus signal. The maximum power tracking system can use redundant shared buses for fault tolerance eliminated a single point failure of the shared bus. The use of the two shared buses is an add on feature without requiring internal circuit modifications of the parallel connected DC-DC converters. These and other advantages will become more apparent from the following detailed description of the preferred embodiment.